In hammer-type pile drivers, the driving force on the pile is derived from the mass of the hammer and the hammer velocity at impact. In any given pile driving operation, the mass of the hammer is fixed, and is usually relatively small for the sake of convenience. Thus, hammer velocity is the primary variable parameter for altering the force applied to the pile. Because there is a limit upon the ability of a pile to accelerate upon acceptance of a blow from a hammer, there is also a practical upper limit in any given situation on the impact velocity of the hammer. If this upper limit is exceeded, the pile will be deformed or damaged by the hammer blow, since it is unable to accelerate out of the way of the hammer rapidly enough. There thus results a practical limit on the driving force available per hammer blow, and on the capacity of the unit.
Most pile driving equipment involves, in addition to the moving hammer, considerable additional structure having, in the aggregate, an appreciable mass. But in conventional pile driver arrangements, this mass is left unexploited as a source of direct pile driving force.